Szathmáry László honlapja @ DEIK | Nim2 / Exception handling

Exception handling in Nim is very similar to Python.

Here is a short example. In an infinite loop, we read two numbers, divide them and print the result. The user can quit with Ctrl+c or Ctrl+d. A lot of things can go wrong; the problems are handled with exceptions.

import std/strutils # strip, split, join
import std/strformat # &"Hello {name}!"
import std/rdstdin # for inputExtra()

type
ZeroDivisionError = object of CatchableError

proc inputExtra(prompt: string = ""): string =
var line: string = ""
let val = readLineFromStdin(prompt, line) # line is modified
if not val:
raise newException(EOFError, "abort")
line

proc main() =
while true:
try:
let
a = inputExtra("1st number: ").parseInt
b = inputExtra("2nd number: ").parseInt

if b == 0:
raise newException(ZeroDivisionError, "division by zero")

let result: float = a / b

echo &"Result: {result:.2f}"
except ValueError:
echo "Error: provide numbers"
except EOFError: # if Ctrl+c or Ctrl+d was pressed
echo "bye"
break
except ZeroDivisionError as e:
echo "Error: ", e.msg # Error: division by zero
except: # catch anything
echo "Error: something went wrong"
#
echo "---"

# ############################################################################

when isMainModule:
main()

$ ./main
1st number: 5
2nd number: 2
Result: 2.50
---
1st number: 2
2nd number: hello
Error: provide numbers
---
1st number: 2
2nd number: 0
Error: division by zero
---
1st number: <Ctrl+C>
bye

You can quit from the program with Ctrl+c or Ctrl+d.

Create your own exception

# create an exception called ZeroDivisionError
type
ZeroDivisionError = object of CatchableError

# throw an exception of this type
if b == 0:
raise newException(ZeroDivisionError, "division by zero")

# catch it and get its message ("division by zero")
except ZeroDivisionError as e:
echo "Error: ", e.msg # Error: division by zero

Exception handling

Create your own exception

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